The Collaborative International Dictionary of English v.0.48:

Trepan \Tre*pan"\, n. [F. tr['e]pan (cf. Sp. tr['e]pano, It.
   trepano, trapano), LL. trepanum, fr. Gr. ? a borer, auger,
   trepan, fr. ? to bore, ? a hole. Cf. Trephine.]
   1. (Surg.) A crown-saw or cylindrical saw for perforating the
      skull, turned, when used, like a bit or gimlet. See
      Trephine.
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   2. (Mining) A kind of broad chisel for sinking shafts.
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The Collaborative International Dictionary of English v.0.48:

Trepan \Tre*pan"\, v. t. & i. [imp. & p. p. Trepanned; p. pr.
   & vb. n. Trepanning.] [Cf. F. tr['e]paner. See Trepan a
   saw.] (Surg.)
   To perforate (the skull) with a trepan, so as to remove a
   portion of the bone, and thus relieve the brain from pressure
   or irritation; to perform an operation with the trepan.
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The Collaborative International Dictionary of English v.0.48:

Trepan \Tre*pan"\, n. [See Trapan.]
   1. A snare; a trapan.
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            Snares and trepans that common life lays in its way.
                                                  --South.
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   2. a deceiver; a cheat.
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            He had been from the beginning a spy and a trepan.
                                                  --Macaulay.
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The Collaborative International Dictionary of English v.0.48:

Trepan \Tre*pan"\, v. t.
   To insnare; to trap; to trapan.
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         Guards even of a dozen men were silently trepanned from
         their stations.                          --De Quincey.
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WordNet (r) 3.0 (2006):

trepan
    n 1: a surgical instrument used to remove sections of bone from
         the skull [syn: trepan, trephine]
    2: a drill for cutting circular holes around a center
    v 1: cut a hole with a trepan, as in surgery